Cancer is a generic name for a wide range of cellular malignancies characterized by unregulated growth, lack of differentiation, and the ability to invade local tissues and metastasize. These neoplastic malignancies affect, with various degrees of prevalence, every tissue and organ in the body. Historically, cancers have been diagnosed using conventional histological and clinical features of the affected tissue or organ. However, it is now apparent that tumors, even from the same tissue or organ, are heterogeneous on the cellular and/or molecular level. As one consequence, the prognosis and/or responsiveness to therapy of each patient may differ. This unpredictability confounds treatment selection and may expose patients to the risks and discomforts of unneeded therapies.
EGFR-positive cancers offer a case in point. EGFR and its downstream signaling effectors, including members of the Ras/Raf/MAP kinase pathway, play an important role in both normal and malignant epithelial cell biology (Normanno et al., Gene, 366:2-16, 2006). Amplification and/or mutation of the EGFR gene and/or EGFR protein overexpression have been associated with various malignancies, including breast cancer, lung cancer, colorectal cancer, ovarian cancer, renal cell cancer, bladder cancer, head and neck cancer, glioblastoma, and/or astrocytoma. Increased EGFR activity (whether as a result of abnormally high protein expression, dysregulation of receptor activity, or other mechanism) is believed to contribute to carcinogenesis. Consequently, EGFR is an established target for therapeutic development.
Several EGFR inhibitors are available for clinical treatment. These include EGFR-specific antibodies (e.g., cetuximab (ERBITUX™) and panitumumab (VECTIBIX™)) and small molecular tyrosine kinase inhibitors (e.g., gefitinib (IRESSA™) and erlotinib (TARCEVA™)). While these treatments have benefited subsets of cancer patients, responses to the drugs are variable. For example, three clinical studies of patients with advanced colorectal cancer using cetuximab in a monotherapy setting and/or in combination with irinotecan (a chemotherapeutic agent) demonstrated response rates of 10.5% or 10.8% for cetuximab alone and 22.5% or 22.9% for the combined therapy (reviewed by Iqbal and Lenz, Cancer Chemother. Pharmacol., 54(Suppl. 1):S32-39, 2004). Similarly, about 10% or about 20% of non-small cell lung cancer (“NSCLC”) patients treated with 250 or 500 gefitinib per day, respectively, responded to the drug and exhibited improved symptoms (Birnbaum and Ready, Curr. Treat. Options Oncol., 6(1):75-81, 2005).
Patient responses to EGFR inhibitors have been correlated with various EGFR metrics. For example, EGFR expression (as measured by immunohistochemistry) was associated with an objective response to erlotinib treatment in NSCLC patients (Tsao et al., N. Engl. J. Med., 353:133-144, 2005). However, survival after treatment in these patients was not influenced by EGFR expression, the number of EGFR copies, or EGFR mutation (Tsao et al., N. Engl. J. Med., 353:133-144, 2005). In both preclinical and clinical settings, somatic mutations in the EGFR tyrosine kinase domain were found to correlate with sensitivity of NSCLC patients to gefitinib and erlotinib but not to cetuximab (Janne et al., J. Clin. Oncol., 23:3227-3234, 2005). Clinical studies of gefitinib demonstrated an association between increased EGFR copy number, mutational status, and clinical response in advanced NSCLC (Cappuzzo et al., J. Natl. Cancer Inst., 97:643-655, 2005).
EGFR antibodies in clinical use (e.g., cetuximab (ERBITUX™) and panitumumab (VECTIBIX™)) bind to the extracellular domain of the EGFR. This receptor domain includes the ligand binding site and these antibodies are believed to blocking ligand binding; thereby, disrupting EGFR signaling. As a result of the therapeutic utility of such EGFR antibodies, many subsequent studies have focused on the production of antibodies (or other binding molecules) specific for the EGFR extracellular domain (see, e.g., U.S. Pat. Nos. 5,459,061, 5,558,864, 5,891,996, 6,217,866, 6,235,883, 6,699,473, and 7,060,808; European Pat. Nos. EP0359282 and EP0667165).
Less attention has been paid to antibodies specific for the EGFR cytoplasmic domain, particularly for therapeutic purposes. However, for example, Hyland et al. proposed the intracellular expression of single-chain antibodies (e.g., scFvs) as a promising approach for selective interference with EGFR signaling. Others have described antibodies specific for the EGFR intracellular domain at least for detection purposes (e.g., Lin et al., Cell. Mol. Immunol., 1(2):137-141, 2004; Hyland et al., Oncogene, 22(10):1557-1567, 2003, Panneerselvam et al., J. Biol. Chem., 270(14):7975-7979, 1995; Gullick et al., J. Pathol., 164(4):285-289, 1991; Dazzi et al., Anal. Cell. Pathol., 3(2):69-75, 1991), and some antibodies specific for the EGFR intracellular domain are commercially available (e.g., Epitomics (Burlingame, Calif., USA), Cat. Nos. 1902-1 and 2235-1; Cell Signaling Technologies (Danvers, Mass., USA), Cat. Nos. 4405 and 2239; Spring Bioscience (Fremont, Calif., USA), Cat. No. E2451).
At least one study compared the reactivity of antibodies specific for the EGFR external and internal domains in the same set of lung cancer tissues (Dazzi et al., Anal. Cell. Pathol., 3(2):69-75, 1991). No significant differences in the reactivity of these antibodies were observed.
The continued clinical development of EGFR inhibitor therapies would benefit from a parallel strategy for identifying patient populations most likely to respond to such treatments. New prognostic and predictive markers, which would facilitate an individualization of therapy for each patient, are needed to accurately predict patient responses to treatments and help clinicians distinguish among treatment choices for such patients.